1. Field of the Invention
The present invention relates to a letterpress printing plate.
2. Description of the Related Art
Letterpress printing is a printing method utilizing recessed and raised parts of a plate, wherein non-image areas are recessed and image areas are raised, and ink applied to the raised parts is transferred to a substrate made of synthetic resin film, paper, etc.
Such letterpress printing methods includes typographic printing that uses a plate with the combination of movable types, photoengraving, line photoengraving and the like, polymer letterpress printing that uses a photopolymer plate in place of a typographic printing plate, and flexography that uses a plate of rubber or photopolymer and supplies a press plate with ink via an anilox roller.
At present, polymer letterpress printing, flexography and rubber plate printing using printing plates formed of a material having flexibility are the mainstream methods in letterpress printing.
In printing plates used for these printing methods, recessed and raised parts corresponding to the patterns to be printed are created by photolithography, laser processing or other techniques.
For example, a letterpress printing plate formed by a photolithography technique is produced by steps of: forming a mask corresponding to a print pattern to be printed on a photopolymer plate; irradiating the photopolymer plate with light via the mask and expose the same to light; and removing the mask and developing the photopolymer plate (see, for example, Japanese Patent Application Laid-Open No. 2005-301071).
In this type of letterpress printing plate, two kinds of raised parts are formed: raised parts for solid shade, whose entire surface transfers ink to a printing substrate such as synthetic resin film or paper, and raised parts for halftone shade, by which so-called halftone dots are printed. These two kinds of raised parts are commonly used in printing plates for resin letterpress printing, flexography and rubber plate printing.
When such a letterpress printing plate is wounded on a cylindrical roll of a printing machine to conduct printing, the same pressure is applied to raised parts for solid shade and raised parts for halftone shade. Hence, if higher priority is given to obtaining excellent printing quality in solid shade areas, halftone areas will be printed in a squashed manner. If higher priority is given to obtaining excellent printing quality in halftone areas, solid areas may sometimes be printed blurrily.
In other words, contact area of per unit area of a solid part is larger than contact area per unit area of a halftone part. As such, when the same pressure per unit area is applied to both a halftone part and a solid part, the pressure per contact area is larger in the halftone part than in the solid part.
Therefore, if higher priority is given to obtaining excellent printing quality in solid parts (when a pressure appropriate for solid parts is applied), excess pressure is applied to halftone parts, so that the halftone parts will be printed squashed (spreaded too much).
On the other hand, if higher priority is given to obtaining excellent printing quality in halftone parts (when a pressure appropriate for halftone parts is applied), the pressure is insufficient for solid parts, and thus the solid parts will be printed blurrily.